The present invention relates in one aspect to a method for positioning the rotor in a downhole progressive cavity pump prior to its use. In another aspect, apparatus connected to the pump stator is capable of resisting downward thrust of the rotor, thereby relieving tension in the supporting rod string.
A progressive cavity pump is located within an oil well, positioned at the bottom end of a production tubing string which extends down the bore of the well. The pump forces fluids up the bore of the tubing string to the surface. The pump comprises a pump stator hung at the end of the tubing string, and a rotor which is both suspended and rotationally driven by a sucker rod string extending downwardly through the tubing string""s bore.
The rotor is a helical element which rotates within a corresponding helical passage in the stator.
The rotor rotates in the stator and drives fluids upwardly. A downward reaction force is created, driving the rotor downwardly. Further, the rotor hangs from the rod string. The combined force of the rod string weight and the pumping reaction causes the rod string to be under significant tension. As a result, the long length of elastic sucker rod stretches and the bottom end of the rotor moves slightly downhole to a lower elevation, perhaps even extending out of the bottom of the pump.
Conventionally, a stop is located at the bottom of the stator and is used to set the initial elevation of the rotor. This stop is known as a tag bar, which extends across the pump""s inlet, minimally impeding the fluid inlet and prevents the rotor from exiting the stator. Once the rotor contacts the tag bar (indicated by a lessening of the suspended weight), then the rod string and rotor are lifted slightly (usually about 1 foot-1xc2xd feet) in anticipation of providing enough clearance that, even when elongated during operation, the rotor will not contact the tag bar again.
Often however, the estimate of dynamic stretch is often inaccurate and the rotor ends up contacting the tag bar during operation anyway. The result is rotating to stationary, metal-to-metal damage; both to the tag bar and possibly to the rotor.
Even if the tag bar and rotor do not contact one another, another result of high tension is the natural tendency of the rod string to assume the straightest possible path, regardless of the profile of the tubing string. The result is a large amount of wear on the tubing string wherever the rod contacts a deviation in the tubing, such as through curves. Rotation stabilizers and centralizers are some of the prior art devices which passively deal with rod tension, deviation and wear.
In U.S. Pat. Nos. 5,209,294 and 5,725,053, both to J. Weber, rotor placer apparatus is disclosed which is located about 30 feet above the pump and which both suppresses vibration from the eccentric rotor and absorbs the downward thrust from the rotor. One perceived difficulty these devices is the quantity of hardware provided and its impact on the flow passages, restricting flow of fluid up the tubing string.
So, there are several demonstrated disadvantages associated with the use of progressive cavity pumps, in particular, due to the movement of the rotor associated with the pumping reaction and the known prior art remedies provided to date.
A supportive platform is positioned beneath the bottom of the rotor of a downhole progressive cavity pump. The platform can rotate, supported by a thrust bearing which is itself supported by a tensile housing secured to the bottom of the stator. Thereby, rotor-imposed loads on the platform translate into tensile loads directed into the stator and tubing string, lessening the load on the rod string and thereby both reducing the need for extreme accuracy in rotor positioning and reducing rod string and tubing wear. Preferably, a no-turn tool transmits load from the stator into the casing.
In one broad aspect of the invention, a rotating tag bar is provided comprising a housing connected to the bottom end of the stator which contains a platform which supports the bottom of the rotor and transferring at least a portion of the weight of the rotor and rod string into a bearing assembly and thus into the housing, stator and tubing string.
Preferably, the platform is a sleeve having a bore which is sized to accept a concentric prong fitted to the bottom of the eccentric rotor. The prong extends part way into the sleeve""s bore and then bears against a stop. The preferred sleeve is supported in the housing using at least one thrust bearing and stabilized using radial bearings. The prong is formed with flow passages to pass well fluid through the sleeve""s bore.
The above apparatus results in a novel method of landing a screw pump rotor suspended from the end of a rod string, comprising: fitting the bottom of a stator with a rotating tag bar apparatus as described above; running in the rotor until its bottom end tags the sleeve stop; and lifting to position the rotor""s bottom end off of the sleeve""s stop a distance which is less than the anticipated stretch, so that when operated and the rotor and rod string, the bottom end of the rotor is supported by the rotating tag bar, lessening the load in the rod string. The positioning of the rotor before operation is no longer critical as contact is encouraged.